Non-linear dependence of interstitial fluid pressure on joint cavity pressure and implications for interstitial resistance in rabbit knee.

Abstract Aims: Synovium retains lubricating fluid in the joint cavity. Synovial outflow resistance estimated as dP [sub j] /d&formmu1; (P [sub j] , joint fluid pressure and &formmu2; trans-synovial flow) is greater, however, than expected from interstitial glycosaminoglycan concentration. This study investigates whether subsynovial fluid pressure increases with intra-articular pressure, as this would reduce the estimated resistance estimate. Methods: Interstitial fluid pressure (P [sub if] ) was measured as a function of distance from the joint cavity in knees of anaesthetized rabbits, using servo-null pressure-measuring micropipettes and using an external ‘window’. Joint fluid pressure P [sub j] was either endogenous (-2.4 ± 0.4 cmH[sub 2] O, mean ± SEM) or held at ∼4, 8 or 15.0 cmH[sub 2] O by a continuous intra-articular saline infusion that matched the trans-synovial interstitial drainage rate. Results: At endogenous P [sub j] the peri-articular P [sub if] was subatmospheric (-1.9 ± 0.3 cmH[sub 2] O, n = 19). At raised P [sub j] the P [sub if] values became positive. Gradient dP [sub if] /dx was ∼20 times steeper across synovium than subsynovium. P [sub if] close to the synovium–subsynovium border (&formmu3;) increased as a non-linear function of P [sub j] to 1.4 ± 0.2 cmH[sub 2] O (n = 23) at P [sub j] = 4.3 ± 0.1 cmH[sub 2] O : 2.3 ± 0.2 cmH[sub 2] O (n = 17) at P [sub j] = 7.6 ± 0.2 cmH[sub 2] O: and 3.0 ± 0.4 cmH[sub 2] O (n = 26) at P [sub j] = 15 ± 0.2 cmH[sub 2] O (P = 0.03, anova). Conclusions: Synovial resistivity is ∼20× subsynovial resistivity. The increase in &formmu4; with P [sub j] means that true synovial resistance d(&formmu5;)/d&formmu6; is overestimated 1.5× by dP [sub j] /d&formmu7;. This narrows but does not eliminate the gap between analysed glycosaminoglycan concentration, 4 mg ml[sup -1] , and the net interstitial biopolymer concentration of 11.5 mg ml[sup -1] needed to generate the resistance. [ABSTRACT FROM AUTHOR]